GIF has antiquated compression and it's a complete waste of bandwidth. Even lossy GIF is worse than lossless optimized PNG.

另外，JPEG/WebP 還是比較小，不過 JPEG 有很多格式，瀏覽器與作業系統的支援度還是很大的阻礙：

Whether lossy PNG gives better results than JPEG depends on the image. JPEG often gives smaller files, except when image has sharp edges (e.g. text) or any transparency (which JPEG does not support at all).

Optimized lossy PNG is still a bit larger than lossy JPEG-XR/WebP/JPEG-2K, but unlike these formats it's supported by all browsers and operating systems without any fuss or hacks.